I have a 2001 f150 with the 4.6 and it has 181,000 miles and my truck was having a miss fire while driving around 60-65mph the when idling the truck feels like it wants to stall/shut off and no power. I had the sparks plugs changed with motor craft plugs the coils are good. I had changed the fuel filter this weekend and it seems a little better. but before I got my throttle body ported and polish it had a lot of carbon build up in there and I never changed the egr valve. and should I check that. there are no codes pending and no engine light on

 

it seems to be worse when the engine is at operating temperatures and it is good when engine is cold

 

I'd say look real carefully under the hood for any damaged vacuum lines first.

From your second post there, it sounds like a small leak. Don't really have enough info to say in what. Could be a vacuum line, could be a gasket. Regardless of what it is, at colder temperatures, it contracts and would likely seal a small hole or tear, but at operating temperature it will expand and allow it to leak.